Thursday, 12 January 2012

Through my work as an SAIS observer, I get to watch the evolution of the snowpack through the winter. Having recently noticed a thread on snowpatch survival over the summer at http://www.winterhighland.info/forum/read.php?2,143591 has already started, I decided to put down some of my own thoughts on the subject.

I often have a look, and have a reasonable photographic record of Coire an Lochan on Aonach Mor. This high East facing coire collects a considerable depth of snow, and where, after a snowy winter, patches sometimes survive through the summer to be incorporated into the following winters snowpack. The last time this happened it this location was after the snowy winter of 2008. The photo below shows South Buttress in Coire an Lochan on 22nd of August 2009, and large patches can still be seen. Over on Ben Nevis snowpatches survive most summers.

Large snowpatches remaining in Coire an Lochan on the 22nd of August 2009.

Although a generally mild late autumn and early winter, it has been quite snowy on the hills of Lochaber. After a mild November, December was about average temperature wise, but with above average precipitation. Heavy periods of snow were accompanied by strong Westerly winds. Before Christmas most of the posts above routes like Morwand and Left Twin were buried, something I had not seen before in 5 winters of working in Lochaber. Although they have reappeared now, it does indicate that some pretty substantial snowfalls occurred. On the other hand there is an almost complete lack of snow on the West face of Aonach Mor, showing how Westerly dominated the winds have been.

There have however, been some pretty savage thaws. For example on both Christmas day and Boxing day summit temperature were around + 4°C and heavy and persistent rain fell. These thaws significantly altered the appearance of the hills, making them look like there was little snow left. However, how much damage did these thaws do in the locations where snowpatches survive well into the summer? Well a thaw will have two affects on the snowpack. Firstly some of the snowpack will melt (a bad thing), and secondly remaining snowpack will compact down and become denser, and after it refreezes become more resistant to further thaws (a good thing). It is thought that the snowpack melted more rapidly than normal after the long cold winter of 2009/10, because it had not gone through many freeze thaw cycles, so was not as dense and icy as usual when the summer came. No problems with that this year.

Coire Dubh on Aonach Mor on the 5th of January this year, I reckon that there is more snow in here at the moment that at any point last winter.

It it difficult to know how much snow actually melted, but I would estimate about a metre. Now this is a pretty small amount as compared to the depths that builds up in the areas where snowpatches survive (maybe 10 or 15 metres in some places). What is left is now dense, icy and granular, making it much more resistant to future thaws, and slower to melt during the summer. Overall I suspect to some extent these factors will cancel each other out, and that these thaws probably didn't have a significant affect on the long term prospects of the snowpatchs this summer.

Anyway I thought I would compare the volume of snow in Coire an Lochan the other day (picture at top of page), with that in spring of other years.

So firstly last year. The picture above was taken on the 5th April 2011. As can be seen there is similar amounts of snow about as at the moment.

This one was taken on the 10th of April 2010, and again quite similar to what is there at the moment.

This one was taken on the 5th of April 2009. We are currently a bit behind.

This one was taken on the 20th of March 2008, a fair bit snowier than at the moment.

Although I don't have such good photographic records, the area where the Aonach Beag snow patches survives many years tells a similar story. The amount of snow there at the moment is similar to that in April 2010, a year when the Aonach Beag patch did survive the summer.

It is currently only early/mid January, plenty of time for further accumulations, hopefully approaching or even surpassing what was about in March/April 2008. As I type this (the evening of the 11th of Jan) the freezing level is above the summits, and it is raining at all levels. It is meant to turn colder tomorrow, but dry with no fresh snow for a few days. However, remember that even during the snowy winter of 2008, there were massive thaws, and times when high pressure kept it cold and dry. So given all this I am quite confident of further snow periods, and optimistic about the chances of the snowpatches this summer.

About Me

Generally based in Fort William I do a variety of different jobs; oil flow engineer, countryside ranger, avalanche forecaster etc. I am interested in snow, and like cutting it up to see how it fractures. I also like drinking tea, climbing and generally pottering about the hills.